Frequency independent non-resonant series fed slot antenna

ABSTRACT

An interferometer antenna comprises upper and lower arrays, each having a non-resonant, terminated, center fed series feed with slot or hole coupling into individual horn radiators nonresonantly spaced somewhat off a half wavelength apart, each half of each array being at an angle other than 180* with the other half. Each half of the upper and lower array is fed with a power divider located near the apex of the array which equally divides the energy between the left and right hand halves of the array. Non-resonant horn spacing and the angular disposition of the array halves provides a beam in the far field along the bisector of the angle between the right and left halves, independent of variations in frequency over useful frequency ranges.

United States Patent 1 1 Goldstone et a1.

7 Aug. 21, 1973 [7 5] inventors: Lenrod L. Goldstone; John 1-]. Cross,

both of Norwalk, Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: Mar. 28, 1972 [21] Appl. No.: 238,854

[51] Int. Cl. H0lq 13/00 [58] Field of Search 343/776, 777, 778, 343/809, 844, 786

[56] References Cited UNITED STATES PATENTS 2,689,303 9/1954 Risser 343/778 2,894,261 7/1959 Yaru 343/776 2,438,735 3/1948 Alexanderson 343/778 Primary Examiner-Eli Lieberman Attorney-Melvin Pearson Williams 57 ABSTRACT An interferometer antenna comprises upper and lower arrays, each having a non-resonant, terminated, center fed series feed with slot or hole coupling into individual horn radiators non-resonantly spaced somewhat off a half wavelength apart, each half of each array being at an angle other than 180 with the other half. Each half of the upper and lower array is fed with a power divider located near the apex of the array which equally divides the energy between the left and right hand halves of the array. Non-resonant horn spacing and the angular disposition of the array halves provides a beam in the far field along the bisector of the angle between the right and left halves, independent of variations in frequency over useful frequency ranges.

6 Claims, 13 Drawing Figures FREQUENCY INDEPENDENT NON-RESONANT SERIES FED SLOT ANTENNA BACKGROUND OF THE INVENTION i. Field of Invention This invention relates to radar antennas, and more particularly to a series fed multi-radiator antenna having a beam direction which is substantially independent of frequency changes.

2. Description of the Prior Art As is known in the art, a narrow radar beam is usually desired so as to pinpoint exactly the location of a target which reflects radar energy back to the radar system. If the beam is very wide, the location of the target cannot be resolved within the beamwidth. It is known that the beamwidth of an antenna pattern (both as to the transmitted beam and the reflections received therefrom) is inversely proportional to the size of the aperture of the radiating element. Some radiating elements comprise large dish-like reflectors which are fed by a horn in front of the reflector (in a manner quite similar to an automobile head lamp). In such case the aperture size is the diameter of the large reflector. Other radiators may comprise arrays of horns which are fed off the end of a waveguide, or through slots or other holes in a waveguide to which the horns are attached. In such a case, the opening of the bell-end of the horn array comprises the aperture and determines the beamwidth.

One type of radar system known to the art is a phase interferometer radar which can measure angles of incident radiation at a pair of radar receiving antennas which are displaced from one another in the plane in which the angle is to be measured. Thus, if two antennas are spaced one over the other, angles in elevation can be measured (either above or below the boresight of the duplex antenna system). Such radar systems are used, for instance, in terrain avoidance systems of the type employed in modern all-weather attack aircraft. In order to provide such an interferometer with each antenna having a narrow beamwidth in azimuth, the use of very large aperture radiators to achieve narrow azimuthal beamwidth would require antennas sufiiciently large that their central spacing would be more than a half a wavelength from each other, resulting in ambiguity in the angle resolution capability of the system. Therefore, it has been known to provide two antennas which are comprised of a plurality of radiating elements along a horizontal line, thus providing an aperture which is physically very wide in azimuth although very narrow in elevation; the elevation beamwidth is immaterial since the interferometry (angle resolving) between the two antenna arrays provides the same resuit as a radar beam which is narrow in elevation.

Phase interferometers known to the prior art frequently must be capable of operating across a range of frequencies, to avoid interference at one frequency, to avoid jamming or other countermeasures, or to permit varying the response of the system to enhance certain targets, and so forth. Therefore, the nature of the antenna, and particularly its feed, must take into account varying frequencies over which the radar system must work.

One way known to the prior art of feeding a plurality of radiating elements (such as horns) on a multielement array of the type used in phase interferometers is referred to herein as a corporate feed. In such a feed, a single waveguide is branched out successively so as to feed two waveguides, the two waveguides feeding four waveguides, and so forth, in a binary fashion, to feed a total of 16 or 32 radiators. This requires a great number of power splitters and/or quadrature hybrid devices which are not only very expensive, but are also heavier than is desired in airborne radar equipment.

An alternative to this is a series feed in which the various radiating elements are disposed along a waveguide, and the signal is fed into one end of the waveguide feeding each of the radiating elements (such as slots feeding horns disposed along the waveguide). If the radiating elements are disposed resonantly, one half wavelength apart from each other along the feed, with a quarter wave short at the end of the array, the resulting beam is broadside or perpendicular to the waveguide, and small changes in frequency do not cause beam scan. However, with frequency changes as large as is required in many systems, the beam will split. Another disadvantage of a resonant series feed is that the input admittance is a rapidly-varying function of frequency, and the input voltage standing wave ratio becomes too high at the frequency band edges of a multifrequency radar to be of any use.

On the other hand, in a simple non-resonant series feed (with the radiating elements spaced uniformly at other than a half wavelength apart and with a lossey termination at the end of the feed beyond the last radiating element), the voltage standing wave ratio remains low over a wide frequency range. However, the beam scans widely with frequency and is therefore impractical where a non scanning beam is required.

SUMMARY OF INVENTION The primary object of the present invention is to provide a simplified feed for a multi-element radar antenna array.

According to the present invention, a non-reasonant series fed multiple element antenna array is comprised of two halves which, in the plane of both of them, are relatively disposed at other than to each other.

The present invention utilizes the interference between the radiation patterns of two, mutually angled halves of a multi-slot array to provide a combined beam which is along the direction of the bisector of the angle between the two antenna array halves, despite the scanning of the beam at each half of the array as afunction of changes in frequency.

' Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawmg.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified, illustrative view of a corporate feed known to the prior art;

FIG. 2 is a simplified, illustrative view of a nonresonant series feed in accordance with the present invention;

FIGS. 3-5 are simplified illustrations of individual beam patterns in the embodiment of an antenna in accordance with the present invention as shown in FIG.

FIGS. 6-8 are simplified illustrations of combined beams of an antenna in accordance with the present invention;

FIGS. 9-11 are simplified illustrations of individual beam patterns in another embodiment of the invention; FIG. 12 is a top plan view of an exemplary antenna in accordance with the present invention; and

FIG. 13 is a front elevation view of the exemplary antenna of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENT The advantage of the present invention is emphasized by comparing FIG. 1, which shows a corporate feed of the type known to the prior art with FIG. 2, which shows a series fed, non-resonant antenna in accordance with the invention. In FIG. 1, a plurality of antenna radiators 20 are shown being fed by a hierarchy of magic tees 22 interconnected by suitable waveguides or trans mission lines and being fed from a single waveguide or transmission line 24. Thus, as described hereinbefore, l couplers 22 are required, rendering the structure expensive and heavy. In contrast, an antenna in accordance with the present invention, as schematically illustrated in FIG. 2, comprises a single magic tee or power coupler 26 being fed by a waveguide 28 and in turn providing half the power of the waveguide 28 into each of two waveguides 30, 32. Each of these waveguides has a plurality of horns 34 disposed thereon, and each terminates in a lossey termination 36, 38 which may comprise a ferrite or may preferably comprise a carbon impregnated plastic. The lossey terminations 36, 38 are provided simply to compensate for the lack of a match at the end of the waveguides 30, 32 and thereby to avoid reflections in the waveguides. The horns 34 are fed by energy coupling apertures, such as holes or slots, spaced along the waveguides 30, 32 by a distance, d, which is slightly greater than a half a wavelength. Because of this, the antenna acts as a true non-resonant series fed antenna, so that the voltage standing wave ratio is very low, over a suitably wide range of frequencies to accommodate the various frequencies required in current radar systems.

A characteristic of non-resonant series fed antenna arrays is that when the spacing of the horns is different than a half a wavelength, there is a tendency for the resultant beam, formed from the beam elements of each of the radiators, to be other than perpendicular to the face of the array of elements. When that spacing is greater than a half wavelength, the resultant beam is ofi-normal in a direction away from the feed-end of the array; when the distance is less than a half a wavelength, the resultant beam is off-normal in adirection toward the feed-end of the array. 1

Thus, as is illustrated in FIG. 3, with horn spacing which is greater than a half a wavelength of a design central frequency (f,,), and with the arrays mutually disposed with an angle of less than 180 in the direction of radiation, the left and right beam patterns (L, R) are parallel to the bisector of the angle between them. For frequencies greater than the design frequency, the left and right beams diverge outwardly from the bisector of the angle between the arrays as seen in FIG. 4. And for frequencies which are less than the design frequency, although still less than the frequency having a wavelength twice as great as the spacings between the horns (herein referred to as f,), the beams converge inwardly towards the bisector, although they would not either of them be normal to the related array. Although the individual beams, as simplistically illustrated in FIGS. 3-5, may diverge and converge with frequencies other than that frequency relating to the angular displacement between the two arrays, the total beam pattern in the far field (as illustrated in FIGS. 6-8) is along the bisector of the angle between the arrays (the boresight of the antenna). As seen in FIG. 6, when operating at the design frequency (FIG. 3) both the right and left beams (R, L) are on the boresight, and the total beam which results from the summation of the individual beams is on the boresight. In FIG. 7, the total beam is still on the boresight even though the right beam is more to the right and the left beam is more to the left, as a result of the individual patterns of FIG. 4, when a frequency higher than a design frequency is used. For a frequency below the design frequency, the same result is achieved as for a frequency above the design frequency. This is illustrated in FIG. 8. The reason is that in the far field (some distance from the antenna) the dimensions of the antenna are extremely small in contrast with the far field pattern. FIGS. 3-8 illustrate that the beam scan ning which results from changes in frequency in a nonresonant series fed antenna array is tolerable provided that two halves of the array are located at an angle with respect to each other which is substantially equal to the angle that the individual beams L, R bear to the antenna. If the angle is too close to then the diverging beams (FIG. 4 will not form a composite pattern as illustrated in FIG. 7. On the other hand, if the angle is too acute (FIG. 5), then the converging beams will cross so as not to form a composite pattern as illustrated in FIG. 8.

The antenna in accordance with the present invention described with respectto FIGS. 2-8 utilizes an angle in the direction of propagation which is less than 180. However, the same result can be achieved by using an angle in the direction of propagation which is greater than 180 as illustrated in FIGS. 9-11. The difference is that the design frequency (f,) is chosen to be less than a frequency having a wavelength which is twice the spacing of the horns. Stated alternatively, the distance is chosen to be less than a half wavelength of the design frequency. This causes the individual beams to be off normal in the direction toward feed. A similar result obtains for frequencies higher than the design frequency (which causes the beams to diverge) and frequencies lower than the design frequency (which causes the beams to converge), as illustrated in FIGS. 10 and 11, respectively.

The detailed design of any antenna in accordance with the present invention is readily determined using antenna design principals which are well known in the art.

An exemplary configuration of an actual antenna in accordance with the present invention is shown in FIGS. 12 and 13. Therein, the feed comprises a flange 40 of the type well known to the art which feeds a magic tee 42 which may include a suitable termination 44 for impedance matching purposes. The magic tee 42 feeds a pair of waveguides 46, 48 upon which respective radiating horns 50, 52 are disposed. The forward walls 54, S6 of the waveguides 46, 48 have apertures, such as slots 58, 60 therein which couple the energy between the homs 50, 52 and the waveguides 46, 48.

In the embodiments of FIGS. 12 and 13, the slots 58, 60 are diagonal slots which provide desired amplitude distribution to the horns. At the output of each waveguide 46, 48 a lossey termination 62 absorbs energy, thereby avoiding reflections which result from the lack of resonance in the waveguide, thereby maintaining a low voltage standing wave ratio. As seen in FIG. 13, the particular antenna array depicted is a dual array having upper and lower radiating elements, such as may be employed in a phase interferometer radar. The lower array 64 is identical to the upper array 38, and in an interferometer arrangement, each have separate feeds of the type described hereinbefore with respect to the upper array 38. if desired, the horns may be flared, sideways, so that adjacent horns are contiguous.

Thus the present invention provides a relatively sim ple series feed which is non-resonant, and therefore, with lossey terminations to terminate each half of a center fed non-resonant series feed antenna array, provides a very low standing wave ratio, and a beam which, within a sufficiently-wide range of frequency, provides a total beam pattern in the far field which is on the boresight of the array. Angles of less than 180 in the direction of propagation are used in systems wherein the radiating element spacing (such as the spacing of slotor hole-fed horns spaced along a waveguide) is greater than a half a wavelength; and angles of greater than 180 in the direction of propagation are utilized when the spacing is less than a half a wavelength. Although described in terms of horns fed by slots or holes in the waveguide to which the horns are disposed, other types of radiating elements, such as slots or dipoles, may be utilized if desired. Similarly, although the invention has been shown and described with respect to preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described typical embodiments of our invention, that which we claim as new and desire to secure by Letters Patent of the United States is:

l. A monopulse dual antenna array comprising:

a pair of antenna sub-arrays disposed mutually adja cent with one another with their boresight in a mutual plane, each of said sub-arrays adapted for operation in a range of frequencies centered about a design frequency, comprising a pair of waveguides disposed in a common plane with the boresight thereof at an angle of other than 180 and between 135 and 225 with respect to each other, each of said waveguides having a lossey termination in a distal end thereof, said waveguides being connected to a common feed at mutually adjacent respective proximal ends thereof, each of said waveguides having a plurality of energy coupling apertures in a wall thereof which is perpendicular to said plane and generally facing in the same direction as said boresight, said apertures being mutually spaced by distances which are other than integral multiples of half-wavelengths but substantially less than one wavelength at said design frequency, and a plurality of radiating antenna horns, one for each of said apertures, each disposed on a related one of said waveguides to radiate energy coupled through the related aperture, each radiating antenna horn on each of said sub-arrays being disposed adjacent a related radiating horn in the other of said sub-arrays.

2. The antenna array according to claim 1 wherein the angle between said radiators, from one line thereof, through boresight, to the other line thereof is less than and the spacing of said radiators is greater than a half wavelength at said design frequency.

3. The antenna array according to claim 1 wherein the angle between said radiators, from one line thereof, through boresight, to the other line thereof is more than 180 and the spacing of said radiators is less than a half wavelength at said design frequency.

4. A monopulse dual antenna array comprising:

a pair of antenna sub-arrays disposed mutually adjacent with one another with their boresight in a mutual plane, each of said sub-arrays adapted for operation in a range of frequencies centered about a design frequency and comprising two groups of individual antenna radiators, each group of radiators disposed along a substantially straight line in a common plane with the other group of radiators, the line of radiators in one group forming an angle of other than 180 with the line of radiators in the other group, within said plane, a pair of transmission lines each having a lossey termination at a distal end thereof and coupled together with the other transmission line to a common feed at a proximal end thereof, said radiators being mutually spaced along said transmission lines by distances other than integral multiples of a half wavelength but substantially less than one wavelength of the design frequency thereof, each of said radiators in one of said sub-arrays being disposed adjacent a related one of said radiators in the other of said sub-arrays.

5. The antenna array according to claim 4 wherein said angle, from one group through boresight to the other group, is less than 180 and wherein said elements are spaced along said transmission lines by distances greater than a half a wavelength.

6. The antenna array according to claim 4 wherein said angle, from one group through boresight to the other group, is greater than 180 and wherein said elements are mutually spaced along said transmission lines by distances less than one-half wavelength of the design frequency of said am 

1. A monopulse dual antenna array comprising: a pair of antenna sub-arrays disposed mutually adjacent with one another with their boresight in a mutual plane, each of said sub-arrays adapted for operation in a range of frequencies centered about a design frequency, comprising a pair of waveguides disposed in a common plane with the boresight thereof at an angle of other than 180* and between 135* and 225* with respect to each other, each of said waveguides having a lossey termination in a distal end thereof, said waveguides being connected to a common feed at mutually adjacent respective proximal ends thereof, each of said waveguides having a plurality of energy coupling apertures in a wall thereof which is perpendicular to said plane and generally facing in the same direction as said boresight, said apertures being mutually spaced by distances which are other than integral multiples of half-wavelengths but substantially less than one wavelength at said design frequency, and a plurality of radiating antenna horns, one for each of said apertures, each disposed on a related one of said waveguides to radiate energy coupled through the related aperture, each radiating antenna horn on each of said sub-arrays being disposed adjacent a related radiating horn in the other of said sub-arrays.
 2. The antenna array according to claim 1 wherein the angle between said radiators, from one line thereof, through boresight, to the other line thereof is less than 180* and the spacing of said radiators is greater than a half wavelength at said design frequency.
 3. The antenna array according to claim 1 wherein the angle between said radiators, from one line thereof, through boresight, to the other line thereof is more than 180* and the spacing of said radiators is less than a half wavelength at said design frequency.
 4. A monopulse dual antenna array comprising: a pair of antenna sub-arrays disposed mutually adjacent with one another with their boresight in a mutual plane, each of said sub-arrays adapted for operation in a range of frequencies centered about a design frequency and comprising two groups of individual antenna radiators, each group of radiators disposed along a substantially straight line in a common plane with the other group of radiators, the line of radiators in one group forming an angle of other than 180* with the line of radiators in the other group, within said plane, a pair of transmission lines each having a lossey termination at a distal end thereof and coupled together with the other transmission line to a common feed at a proximal end thereof, said radiators being mutually spaced along said transmission lines by distances other than integral multiples of a half wavelength but substantially less than one wavelength of the design frequency thereof, each of said radiators in one of said sub-arrays being disposed adjacent a related one of said radiators in the other of said sub-arrays.
 5. The antenna array according to claim 4 wherein said angle, from onE group through boresight to the other group, is less than 180* and wherein said elements are spaced along said transmission lines by distances greater than a half a wavelength.
 6. The antenna array according to claim 4 wherein said angle, from one group through boresight to the other group, is greater than 180* and wherein said elements are mutually spaced along said transmission lines by distances less than one-half wavelength of the design frequency of said array. 